A heat sensitive image recording material comprises a support and a heat sensitive layer that can be colored when a thermal energy is applied to the image recording material. The support is usually made of a paper or a plastic film. The heat sensitive image recording material is well known, and has been commercially used in a facsimile machine or a printer of a computer. The heat sensitive image recording material has an advantage that an image can be quickly recorded using a compact device. The heat sensitive material has another advantage of scarcely polluting the environment. Further, the heat sensitive material is economically advantageous, since the cost of preparing the material is not expensive.
On the other hand, a decoloring type heat sensitive image recording material has recently been proposed. The decoloring type recording material has a colored layer, which can be decolored by a thermal energy to form a negative image. The decoloring type recording material can be advantageously used as a sheet for an overhead projector (OHP) or a hard copy of a CRT image. The hard copy of the CRT image has frequently been used in medical devices, such as a X-ray radiographic device, a supersonic diagnostic device and a CT scanner. The decoloring type recording materials are described in Japanese Patent Provisional Publications No. 59(1984)-229385, No. 62(1987)-13389 and No. 62(1987)-249787.
The known decoloring type heat sensitive image recording material contains a colored dye and a decoloring agent (e.g., a polyethylene glycol derivative, a base). The colored dye is formed by a reaction of a leuco dye with an acidic developer. The leuco dye and the acidic developer are used in conventional heat sensitive or pressure sensitive image recording materials. The colored dye can be decolored by a reaction of the dye with the decoloring agent. The reaction is caused by a thermal energy. Thus, a white or transparent image over a color background is recorded within the heated area.